You may not see them, but they are studying your every move. To identify dangerous people, the Transportation Security Administration (TSA) has stationed specially trained behavior detection officers at 161 U.S. airports. The officers can be positioned anywhere, from the parking garage to the gate, trying to spot passengers who show an unusual level of nervousness or stress.

Seattle Times’s Ken Kaye quotes Sari Koshetz, spokeswoman for the TSA, to say that they do not focus on nationality, race, ethnicity, or gender. “We’re not looking for a type of person, but at behaviors,” she said.

The program started in Boston in 2003 and expanded to other airports. Under the program, a suspicious passenger might be given a secondary security screening or referred to police; detection officers don’t have powers of arrest.

Last year, officers nationwide required 98,805 passengers to undergo additional screenings. Police questioned 9,854 of them and arrested 813. The TSA does not break down the numbers for individual airports.

Kaye writes that it is not easy to spot detection officers. Working in teams of two and clad in TSA uniforms, they blend in with those performing screening chores at the security checkpoint. While they do not require any previous background in behavior analysis, the officers are chosen based on their intelligence, maturity, and ability to work with people, the TSA said.

Officers undergo four days of behavior training, which includes trying to spot suicide terrorists, and then receive twenty-four hours of on-the-job training.

On a recent Monday, two detection officers (their names are not revealed to protect their covert status) eyeballed hundreds of passengers at Florida’s Fort Lauderdale-Hollywood International Airport. After noticing a passenger with red eyes who was holding a large carrying case, Juan started a friendly conversation with the man. Such chats allow an officer immediately to tell whether a traveler has malicious intentions, Juan said.As it turned out, the case contained a musical amplifier and the man was simply tired.“He was pretty calm,” Juan said.

Humberto said he has assessed people as diverse as businessmen in suits and mothers with screaming children. “When somebody’s trying to hide something, it’s going to show,” he said.

Koshetz said the TSA has established specific criteria for what is considered normal behavior “in an airport environment.” She said officers react only if a passenger strays from those guidelines, which the TSA declines to reveal for security purposes.

The observation of passengers does not end in the airport. On an undisclosed number of domestic and international flights, federal air marshals pick up where the behavior detection officers leave off. The marshals blend in with passengers and work covertly to spot suspicious behavior, said Nelson Minerly, spokesman for the Federal Air Marshal Service, which also falls under the TSA. “If the public can’t find us, the bad guys can’t find us either,” said Minerly, an air marshal since 2002.

The exact number of air marshals currently working is secret, but Minerly noted that they are federal law enforcement officers with arrest powers and the authority to use lethal force. If a passenger causes trouble, air marshals have several options. They can ask crew members to help subdue a person. They can wait until the plane lands and call for backup. Or they can draw their weapons.